Double-acting hydraulic cylinders are very widely used to lift loads and keep them in the air. They are frequently used in the fields of transport, industry, building, and any fields where it is necessary to perform lifting, lowering and/or maintaining movements of a load involving significant forces. This is for example the case for auto carrier vehicles, which traditionally includes several moving platforms intended to receive the automobiles to be transported, and the position and height of which can be adjusted using different actuators, including double-acting hydraulic cylinders.
For safety reasons, it is absolutely essential to guarantee the maintenance in position of the cylinder rod when its movement is not controlled, even and especially when it bears a load. Indeed, to guarantee the safety of the operators and goods that may be located below, the height of the load must not vary over time without deliberate action by the operator. To that end, unless controlled, the rod of a hydraulic cylinder must be immobilized inwardly or outwardly, irrespective of whether it bears a load.
In the prior art, a cylinder is traditionally kept under a load by using a check valve inserted into the supply circuit of the cylinder with the passing direction going toward the outlet chamber of its rod. However, this assembly for maintaining a load is only provided if it is possible to guarantee the sealing of the valve. Yet a valve may gradually or suddenly have a sealing defect after a certain usage period, for various reasons, for example related to a construction defect, an operating or maintenance problem, or aging of the valve. Such a sealing defect will cause a leak which, even if limited, may, after several hours of retention, cause a slow lowering of the load, which is dangerous for the goods and people located below.
To offset this risk of failure of the valve, in the prior art, mechanical blocking devices have been developed that are associated with these lifting cylinders. This involves a manual blocking system, for example locking pins. However, these mechanical blocking devices are not automatic and must be put in place manually. A risk of forgetting or of incorrect manipulation by the operator is therefore always possible.
Also known from patent EP 0,935,715 is an automatic maintaining device developed with the aim of eliminating the use of these traditional mechanical retaining means. This device is made up of a hydraulic isolating unit that replaces the traditional check valve mounted in the supply circuit of the cylinder. This hydraulic unit, through which the outlet chamber of the rod of the cylinder is supplied, comprises a primary ball-check valve, which is duplicated by a second so-called “security” valve, closed by a rod conical needle. These two successive valves are each arranged at one of the inlets of a sluice, the closing of the security valve being done in a differed manner relative to that of the primary valve. When the operator controls the opening of the device, a control means with a rod opens the security valve by pushing on the conical needle, which moves in the sluice. Through this movement, the rod of the needle pushes the ball of the primary valve, which opens in turn.
This known device has several advantages relative to the prior systems. First, it is safer than the traditional check valve due to the use of two successive valves instead of only one. Furthermore, it allows automatic locking, without operator intervention and in any position of the cylinder rod.
However, this prior device has certain drawbacks that prevent it from guaranteeing the desired complete security.
With such a device, there is still a risk of leaking increased because of the hyperstatic nature of the device. Indeed, the sliding of the conical needle is guided by its rod, in a tight and hyperstatic manner. Since this rod is connected to the conical needle of the security valve, the positioning of one of these parts conditions the positioning of the other part that is connected to it. If one of them is oriented incorrectly or has a geometric defect, this causes incorrect positioning of the other. In order for the secondary valve to close correctly, the machining must therefore be done with perfect centering and geometry, the slightest perpendicularity defect being able to cause a leak, since contradictory forces are generated. These leaks can appear from the beginning, in case of construction defect, or later, in case of alteration in the geometry for example related to wear or the presence of a bothersome impurity.
Furthermore, this device locks the oil in only one of the chambers of the hydraulic cylinder. No security is provided to prevent leaking of the oil located in the second chamber of the cylinder located on the rod side. The outlet of the rod is therefore not locked.
For all of these reasons, this prior device is not completely secure.